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Above: plant growths caused by the larvae of three different species of gall wasp, including Trigonaspis teres, Callirhytis seminator, and Feron izabellae

These tumor-like growths are known as plant galls. They develop in response to chemicals that are injected or secreted by certain insects, mites, and nematodes. Each plant gall forms around the body of a single larva (or, in some cases, a small group of larvae), and the structure serves as both protection and sustenance for the tiny creature developing within.

Above: the plant gall of the oak apple gall wasp, Atrusca quercuscentricola, with a bisected view that shows the larva within

There are many different insects that can trigger the production of plant galls, including certain aphids, psyllids, flies, beetles, scale insects, and caterpillars, but gall-forming wasps are especially diverse. They also create some of the most distinctive plant galls in nature.

Above: the photo at the top shows the plant gall of an unidentified gall wasp from the family Cynipidae, and the photo at the bottom depicts the plant galls of the urchin gall wasp, Cynips quercusechinus

The color, shape, size, and texture of each plant gall varies depending on the species of gall wasp that induces it. Some wasps are associated with plant galls that look like fuzzy little pom-poms; others produce mushroom-shaped structures, colorful discs, cones, pink spheres, cottonballs, etc.

Above: this photo shows a mushroom gall wasp, Heteroecus sanctaeclarae, which produces plant galls that look like tiny mushroom-shaped houses

As this article explains:

Galls are plant growths (similar to tumors) that are induced by various organisms such as bacteria, fungi, and insects. Gall wasps have evolved to “trick” the plant into forming this growth which they then use for food and shelter as they transform from a larva to an adult.

The wasp larvae secrete chemicals that mimic growth hormones in a particular plant upon hatching. The chemicals trick the oak into growing a gall on its flowers, acorns, leaves, or stems. The larva is then encapsulated by the gall as it grows, waiting patiently inside until its metamorphosis is complete.

Above: Feron parmula, commonly known as the disc gall wasp

Many of these plant galls have elaborate, colorful features that are truly stunning.

Above: the spined-turban gall wasp, Cynips douglasii

Gall-forming wasps are only parasitic toward plants -- they do not parasitize other animals. The larvae feed on the nutritive tissues of their plant galls, but the adult wasps do not feed at all.

Above: plant galls produced by two different species of gall wasp

These wasps also have a peculiar reproductive cycle:

Many species have alternating generations, meaning all of the adults emerging from galls during one time of the year are female-only, while the adults emerging in a different season have both males and females. Most species have females that can reproduce using parthenogenesis when they emerge by themselves. This means that their eggs are essentially clones of themselves. What’s more, some species appear not to have any males at all.

Above: the huge, fuzzy plant galls of Striatoandricus furnessae and Druon pattoni

Scientists have named and described roughly 1400 species of gall wasp, and that's likely just a fraction of the number of species that actually exist, as gall-forming organisms are widely understudied.

Above: close-up of a gall wasp larva nestled in its plant gall

Once the larva transforms into a fully-developed wasp, it finally emerges from its gall.

Above: adult gall wasps

Sources & More Info:

Forest Watch: Gall Wasps

Gallformers: What the Heck is a Gall?

Southwick Country Park Nature Reserve: Ecosystem Engineers

Insect Systematics and Diversity: Comparative Anatomy of Venom Glands Suggests a Role of Maternal Secretions in Gall Induction by Cynipid Wasps

Entomology Today: Gall-Inducing Wasps Have Enlarged Venom Glands, Study Finds

The British Plant Gall Society: Plant Galls